A continuing goal of integrated circuit manufacturers is that of compactness. As the physical size of integrated circuits is decreased, the fabrication of such devices can become more economical. One approach to increasing compactness is to take advantage of processing technology advances. Processing advances have led to generational reductions in minimum achievable feature sizes, and hence the existing size of designs can be "shrunk" to achieve smaller and smaller integrated circuit device sizes. While simple shrinks are effective in reducing the physical size of a device, such approaches are obviously limited by processing technology. Eventually, a minimum size will be achieved until the next processing advance comes into play. Furthermore, as another limitation, smaller feature sizes can require lower operating voltages, as the resulting smaller circuit devices may be incapable of functioning at certain power supply voltages.
Another goal of integrated circuit manufacturers is power reduction. The increasing prevalence of battery powered portable electronic devices has made lower power integrated circuits more desirable. Lower power components directly contribute to the amount of time a device can be operated before its batteries need to be replaced or recharged. The typical approach to reducing power consumption involves reducing the power supply voltage. In this way, generational shrinks can serve the goal of reducing power consumption.
While the use of lower power supply voltages can be implemented in logic circuits with relative ease, this is not necessarily true for more complex integrated circuits. For example, some integrated circuits require multiple supply voltages. Multiple supply voltages can be provided by the system in which the integrated circuit is used (e.g., a system with both a 3.3 volt bus and a 5 volt bus), such an approach is undesirable due to the added space and complexity of the system. A more efficient approach, when possible, is to generate the additional supply voltages on the integrated circuit itself. For example, many types of semiconductor memory devices include an array portion that is operated at a potential that is less than the power supply voltage. Alternatively, the array may be operated at the power supply voltage while the remaining "peripheral" circuits are operated at a potential that is greater than the power supply voltage. Further, in many programmable integrated circuits, such as electrically erasable programmable read-only-memories (EEPROMs), flash EPROMs, and programmable logic devices (PLDs), one or more additional supply voltages may be generated on the device to program and/or erase programmable devices within such integrated circuits. Such non-power supply voltages are typically larger than a high power supply voltage, or lower than a lower power supply voltage (are "outside the rails").
The use of non-power supply voltages on an integrated circuit can require special circuits. For example, the generation of outside the rail voltages is typically accomplished with a charge pump circuit. The charge pump circuit can be a single action circuit which "boots" a selected node with a single transfer of charge, or for the generation of larger magnitude voltages, a number of charge pump stages arranged in series. Each stage providing a progressively larger and larger magnitude voltage.
Another specialized circuit that is employed with non-power supply voltages results from irregularities in such voltages. In the case of non-power supply voltages, as the charge pump circuit "pumps" a node to a higher (or lower) voltage level, it may be necessary to prevent the resulting voltage from exceeding a particular value. As just one example, in the case of metal-oxide-semiconductor (MOS) transistors, the outside-the-rail voltage should be prevented from exceeding a source-drain breakdown voltage, or an oxide breakdown voltage. In addition, the stability of non-power supply voltages is desirable to ensure predictable operation of integrated circuit functions. EEPROM memory cells, for example, require predictable voltages to ensure proper programming and/or erasure of such cells. Consequently, where stability of a non-supply voltage is required, a voltage regulator circuit is provided.
Voltage regulators monitor the magnitude of a non-power supply voltage, and in the event the voltage varies from a reference level, an adjustment is made to bring the voltage back into the predetermined range. For example, in the case of a higher-than-supply voltage, a charge pump circuit may be activated that begins to charge a non-power supply node. In the event the non-power supply voltage exceeds a predetermined reference level, the voltage regulator circuit will begin discharging the non-power supply node to thereby maintain the node at the predetermined level. Typically, higher-than-supply voltages are coupled to the lower power supply voltage. In determining when to discharge a non-supply node, many voltage regulators use a compare operation. The non-supply voltage is continually compared with a reference voltage. When the reference voltage is exceeded, a discharge circuit is activated. Thus, the accuracy of a voltage regulator will usually depend upon the accuracy of its reference voltage.
Generating an accurate reference level can be problematic because of the nature of semiconductor circuit devices (e.g., transistors, resistors, etc.) and the environment in which integrated circuits operate. One particular problem has to do with temperature. As the operating temperature of an integrated circuit varies, the properties of the circuit devices vary.
A prior art way to address temperature related variations in a reference voltage has been to utilize a separate "band-gap" reference circuit to generate the reference voltage. The reference voltage would then be applied to the voltage regulator for the compare operation. Band-gap reference circuits take advantage of the fact that the base-emitter voltage (VBE) of a bipolar transistor has a negative temperature coefficient. That is, as the temperature increases, the VBE of a bipolar transistor will decrease. At the same time, the thermal voltage (VT) of the bipolar transistors, as well as resistor values, have positive temperature coefficients, and so can be used to compensate for drift in the VBE value. The output voltage at which a stable dc reference voltage can be maintained over considerable temperature variation turns out to be in the range of +1.25 volts. Band-gap reference circuits derive their name from this voltage, as it is close to the band-gap voltage of silicon.
While band-gap reference circuits can provide stable reference voltages, a drawback to such approaches is the additional complexity required to implement the circuits. As noted above, band-gap circuits typically require bipolar transistors. This can be problematic in the case of a MOS based integrated circuit, as an additional type of circuit device must be integrated into the fabrication process. In addition, band-gap circuits can require considerable area to implement, adding to the overall size of an integrated circuit.
Another important goal of integrated circuits is that of power supply voltage flexibility. Different electronic devices may have different power supply levels. It is therefore desirable that an integrated circuit be capable of providing the same functionality over a range of power supply voltages. Such a goal can be difficult to achieve if voltage regulator circuits are required, because the reference voltage utilized must not only be temperature independent, but also power supply independent as well. Complex band-gap reference circuits can be capable of providing such a reference voltage, but will have the drawbacks mentioned above.
In light of the increasing use of non-power supply voltages in integrated circuit devices, there is a need for a voltage regulator circuit that is compact, can compensate for temperature variations, but that does not require a mixture of circuit device types. In addition, there is also a need for a voltage regulator circuit that can operate over a range of power supply voltages.